A fuel injector injects atomized fuel so that combustion efficiency is improved and incomplete combustion is avoided. In a case that engine temperature is relatively low, a part of the injected fuel is not vaporized and adheres on a wall of the internal combustion engine as unburned fuel. Due to this unburned fuel, unburned hydrocarbon (HC) and particulate matters (PM) are increased in exhaust gas.
JP-2007-239523A shows a vaporized fuel supply system in which fuel is heated to be adsorbed by a canister. When an engine is started, a purge vale is opened so that the adsorbed fuel is purged into an intake pipe, whereby combustion efficiency is improved and emission is reduced. Further, JP-2003-343365A shows a fuel supply system in which vaporized fuel in a fuel tank is injected by a sub-injector in order to improve combustion efficiency and to reduce emission.
However, in the fuel supply system shown in JP-2007-239523A, it takes long time period to purge the vaporized fuel from the canister. Thus, it is likely that the vaporized fuel can not be supplied to the engine when the engine is started. In the fuel supply system shown in JP-2003-343365A, a pump and a valve are necessary to supply the vaporized fuel from a fuel tank, which increases the number of parts and components. Further, in both fuel supply systems, since the concentration and the pressure of the vaporized fuel are not constant, it is relatively difficult to control the air-fuel ratio precisely.